The increasing need for various electric loads for motor vehicles has increased the need for power supply systems capable of supplying a higher voltage to some of the various electric loads, and a lower voltage to the remaining electric loads.
An example of such power supply systems is disclosed in Japanese Patent Application Publication NO. 2007-153149.
The power supply system disclosed in the patent application Publication includes a lower battery, a higher battery, a DC to DC converter, and an electronic control unit. Under control of the electronic control unit, the DC to DC converter works to boost a DC voltage of the lower battery, and supply the boosted voltage to the higher battery. Under control of the electronic control unit, the DC to DC converter also works to step down a voltage of the higher battery, and supply the stepped-down voltage to the lower battery.
Because the structure of the power supply system requires a costly DC to DC converter, it is difficult to reduce the cost of the power supply system.
Thus, in order to achieve such a different-voltage supplying function without using DC to DC converters, U.S. Pat. No. 6,320,775 corresponding to Japanese Patent Application Publication NO. H10-337047 discloses a power converter system.
The power converter system disclosed in the U.S. Patent includes a DC battery serving as a lower battery, a smoothing capacitor serving as a higher battery, a three-phase inverter, and a three-phase induction motor; these inverter and induction motor serve as loads of the smoothing capacitor. The power converter system is designed to provide the different-voltage supplying function with the use of the three-phase inverter and three-phase induction motor as the loads of the smoothing capacitor.
Specifically, a high DC-voltage terminal and a low DC-voltage terminal of the three-phase inverter are connected to one electrode and the other electrode of the smoothing capacitor, respectively. Any one of the high DC-voltage terminal and the low DC-voltage terminal of the three-phase inverter is connected to one of high-side and low-side electrodes of the DC battery. One end of each phase winding of the three-phase induction motor is connected to a corresponding AC terminal of the three-phase inverter. A neutral point of the three-phase windings is connected to the other of the high-side and low-side electrodes of the DC battery.
The three-phase inverter works to convert a DC voltage across the smoothing capacitor into a three-phase AC voltage, and supply the three-phase AC voltage to the three-phase induction motor so as to activate the three-phase induction motor.
During the three-phase induction motor being activated, the three-phase inverter works to:
boost a DC voltage of the DC battery so as to supply the boosted DC voltage to the smoothing capacitor; or
step down the DC voltage across the smoothing capacitor so as to supply the stepped-down DC voltage to the DC battery.
The power converter system disclosed in the U.S. Patent is designed to boost a DC voltage of the DC battery so as to supply the boosted DC voltage to the smoothing capacitor or step down the DC voltage across the smoothing capacitor so as to supply the stepped-down DC voltage to the DC battery only when the three-phase induction motor is activated.
For this reason, during the three-phase induction motor being inactivated, if power is consumed by another load, it may be difficult for the power converter system to compensate the consumed power. In such a situation, activation of the three-phase induction motor may cause a rush current to be created immediately after the activation based on the DC voltage across the smoothing capacitor, resulting in significantly reducing the DC voltage across the smoothing capacitor. This may make it difficult to stably supply the different DC voltages to the electric loads.